Police evacuated thousands of people and sealed off the center of England's second city Birmingham on Saturday night in the biggest security alert since four bombs exploded in London killing more than 50 people.

Acting in response to what they said was intelligence of a threat, police cleared the city's entertainment and Chinatown districts of some 30,000 people and carried out a controlled explosion on a bus.

But they stressed the security alert was not connected to last Thursday's bomb attacks in London.

Pubs and restaurants were shut and hotels were evacuated as the huge operation swung into action. Police blocked all roads into the center while helicopters flew overhead and ambulances were positioned around the central Broad Street district.

At 6 a.m. (0500 GMT) on Sunday, the operation was still under way but police said they had begun reopening some of the areas to the public.

It was among several security scares in Britain on Saturday in the wake of the London attacks. Police have warned the bombers may still be at large and could strike again.

Earlier, they revealed that the three bombs that ripped through London underground trains went off almost simultaneously, making it more likely they were detonated by timers rather than suicide bombers.

A fourth bomb that blew up a bus almost an hour later was probably left in a bag and not triggered by a suicide bomber, they added.

Investigators struggled in extreme heat to retrieve bodies still trapped underground after the attacks, while anxious relatives frantically looked for loved ones missing since the rush-hour blasts on Thursday morning.

Police said the process of recovering bodies could continue for days in a hot, narrow and rat-infested tunnel deep below ground at King's Cross station.

DEADLIEST ATTACK

The government said people across Britain would be asked to observe two minutes' silence at noon on Thursday.

Police, who have made no arrests, said they were looking for no specific individuals and that the bombs were made of high explosive, not home-made materials.

Three Islamist groups have claimed responsibility for the blasts, which government ministers said bore the hallmarks of the Islamic militant al Qaeda network which was behind the Sept. 11, 2001, attacks on the United States.

"All three bombs on the London Underground system actually exploded within seconds of each other at around 8:50 in the morning," Scotland Yard Deputy Assistant Commissioner Brian Paddick told a news conference on Saturday.

He said the fact that the bombs went off within 50 seconds of each other suggested they were set off by timers rather than detonated manually, although police did not rule that out.

Police previously believed the three blasts were spread over nearly half an hour, but revised their information in the light of new technical data and witness statements, he said.

The fourth bomb exploded on a bus near Euston railway station at 9:47 a.m.

"The most logical explanation is that one of the terrorists was unable to board an underground train -- probably because of the rapid closure of the system -- and ended up with a primed bomb and no target," said Dominic Armstrong, head of research and intelligence at security group Aegis Defense Services.

"In the circumstances, it seems understandable that he should seek another similar target quickly."

Police say there could be more attacks and security experts suggest would-be attackers could opt to kill themselves and take as many victims with them as possible if confronted.